The present invention relates to methods and apparatus for precise position determination and in particular to a combination of electro-optical methods and apparatus and advanced data processing methods and means for the very precise determination of absolute position and incremental displacement of objects.
Prior art electro-optical apparatus for the determination of the position of a rotational objects is shown in FIG. 1. A light-opaque mask 12 having light-transparent apertures 20 is rigidly attached to and moves with an object 22, whose angular position is to be determined. Light rays emitted from a light source 10, impinge on the mask 12 and a light flux passing through the apertures in the plate is detected by a light detecting device 14, which is aligned with the mask and the source to measure the light flux.
As different apertures 20, in the mask 12 pass in front of detector 14, an output signal is obtained from detector 14 which determines the position of object 22 with the aid of a position reporting device 18.
The problems associated with this type of apparatus and this method for position determination relate principally to limitations in position determination accuracy. Best positional accuracy obtained with this method and similar prior art methods is of the order of 1 micrometer.
The reasons for this accuracy limitation can be traced to the fact that light passing through an aperture and detected by a light sensor is registered either as a zero or as a one, i.e., there are no fractional measurements; thus, the positional accuracy is determined only by detection and identification of a particular slit through which light passes. Thus, in order to determine very small distance displacements the aperture width and the aperture intervals must be made impossibly small. Physical size, therefore limits the fine range measurements.
Other systems are capable of finding a position of an edge to greater accuracy than the size of the detector by utilizing digitized values at a number of positions as a single detector moves past the edge. The values are used to find the position of the edge. However, such methods are capable of less than an order of magnitude of improvement of resolution and furthermore, they do not measure the position at a point without making measurements at adjacent points. Thus the position of the edge is known only after the detector has moved away from the edge and is therefor not known in real-time, i.e., while the measurement is being made. Such determination of edges is used, for example, in the inspection of reticules for microcircuit production.
The apparatus of FIG. 1 is also sensitive to vibrations which can lead to large errors in position determination. Such prior art apparatus and similar apparatus (as described for example in Japanese Patent Publication No. He-1-22884) is, thus, not suitable for very precise, high resolution position determination.
For more precise positional accuracy determination, of better than 1 micrometer, most prior art devices rely on optical interferometry, which suffers from high sensitivity to vibration. Furthermore, such interferometers are high cost, very delicate apparatus. Examples of such apparatus are described in the General Catalog of Dr. Johannes Heidenhain, GmbH of Traunreut Germany dated June 1996, in particular on pages 6 and 7.
The object of this invention is to provide techniques, measurement and calibration methods to determine an object""s position and displacement with high resolution and accuracy, better than 0.01 micrometers. This accuracy is at least one order of magnitude better than that quoted for the most accurate linear encoder in the above mentioned catalog and two orders of magnitude better than almost all of the listed encoders. Various preferred embodiments of the invention perform such determination for various object positional displacement conditions and geometries. It is noted that the accuracy of the method and apparatus of the present invention is greater (better) than the diffraction limit of the light to which the detectors are sensitive.
In the realization of the above mentioned object of the invention a linear array of light detectors is used to detect the light received from a striped mask which is attached to an object whose position is to be precisely determined. The mask provides a pattern of bright and dark light strips over the linear detector array surface, when it is suitably illuminated.
The method employed in this invention for positional determination is based on determination of the positions of the boundary lines between a plurality, generally a large number of, bright and dark parallel strips of light obtained from the mask. This positional line information is obtained from the detected light level of each of the light detectors in an array (such as a linear or matrix type CCD array). The output of each individual detector is converted into one of many possible digital values, i.e., a gray scale of values is determined. The line position, and thus the object position, are determined from the values provided by the plurality of detectors.
By averaging or otherwise processing the positional data of the light detectors, high accuracy in positional determination is obtained. Preferably, hundreds or thousands of detectors are used, resulting in improved accuracy.
In a preferred embodiment of the invention, apparatus is provided in which a pattern of alternating bright and dark strips illuminate an array of light detectors. In one preferred embodiment the source of the mask strips is a plurality of alternating light transparent and light opaque strips. In another, embodiment a mask utilizes a plurality of alternating light reflecting (white) and light absorbing strips (black). In some preferred embodiments of the invention the strips are oriented at an angle to the direction of position measurement and provide a pattern of alternating bright and dark angled strips across an array of detectors.
The strips are preferably at an angle to the direction of motion (and to the repeat direction of the cell array) which angle is preferably chosen to optimize the number of linear detecting cells. Linear detecting cells are cells in which the boundary lines between bright and dark strips cross both sides of the detector cell which are generally perpendicular to the direction of motion. In this case the output signal from the cell is linearly related to the fraction of the cell area covered by the bright strip. In most embodiments of the invention, only xe2x80x9clinearxe2x80x9d cells are used in the determination of the position. Generally, the cells are rectangular and the cells are in the linear region and greater accuracy is achieved when the long sides of the detector are perpendicular to the direction of motion.
The detected output signal of each linear cell, is preferably digitized into one of 256 levels which are proportional to the fraction of the cell area illuminated by the bright strip and which enables determination of the bright-dark interface lines position.
Each cell""s fractional illumination information is fed to a signal processor which determines the position of the object. The positional information provided by each xe2x80x9clinear cellxe2x80x9d and by all the linear cells together is used to provide an average position calculation which is more accurate than the positional determination provided by a single cell (typically by at least a factor of 10 and more typically by a factor of 100).
Another aspect of the invention includes the calibration of the sensitivity of each detector cell by moving the subject on which the mask is mounted, so as to allow the area of each cell to be completely illuminated by a bright strip and alternately, to be completely in the shadow of a dark strip, Normalization factors for each cell can be accurately determined.
This normalization is part of a broader aspect of the invention which includes the provision of methods and apparatus for the Auto-Calibration of the light detectors. Such auto calibration is carried out, for example, for each linear detector by measurement and recording of the difference between the position reading as derived from that cell""s light measurement and the positional data determined by averaging the positional outputs of all linear detectors. That difference is recorded, for each cell, in the signal processor and used for correction of each cell""s output and for provision of more accurate positional data.
Other aspects of the invention provide apparatus for the attainment of accurate average object position in the presence of vibrations, by use of time averaging of position measurements. Also provided, in a preferred embodiment of the invention, are means to ascertain the absolute position of the object by use of specially designed and configured strips and coded strip widths.
In another preferred embodiment of the invention, the illumination of the scale (which is either reflective or transparent), is done by a flashing light pulse of short duration, so as to freeze the object (if it moves) at a well-defined moment in time.
In still another preferred embodiment of the invention, means are provided for the attainment of even higher positional accuracy by improved alignment of detectors in relation to the mask strips and by use of repeated positional measurements and of averaging of the obtained positional data.
There is therefore provided in accordance with a preferred embodiment of the invention, apparatus for the measurement of position of an object in a given direction, comprising:
a one dimensional array of light detectors, comprising a plurality of detectors including at least three detectors, each of which produces a signal at a given position in response to light reaching the detector;
at least one edge between a dark area and a light area viewed by at least one of the plurality of the detectors, which the at least one edge moves, in relation to the plurality of detectors, as the object moves in the given direction; and
computing circuitry which receives signals from the at least one detector viewing the at least one edge and determines the position responsive to the signals produced by the at least one detector at the given position.
Preferably, the at least one detector comprises two or more detectors. More preferably, the at least two detectors comprise at least three detectors. Alternatively or additionally, the plurality of detectors comprises at least one detector which views only the bright area and at least one detector which views only the dark area and wherein the computing circuitry receives signals from the at least one detector viewing the edge, the at least one detector viewing the dark area and the at least one detector viewing the bright area and wherein the computing circuitry computes the position responsive to the thus received signals.
Preferably, the position is determined from the ratio between (a) the difference between the signal from the detector which views the edge and the signal which views the bright area and (b) the difference between the signal from the detector which views the bright area and that which views the dark area.
Alternatively or additionally, the at least one edge is perpendicular to the direction of motion. Alternatively, the at least one edge is parallel to the direction of motion. Alternatively, the at least one edge is neither perpendicular nor parallel to the direction of motion.
Alternatively or additionally, the line of detectors is parallel to the direction of motion. Alternatively, the line of detectors is perpendicular to the direction of motion. Alternatively, the line of detectors is neither perpendicular to or parallel with the direction of motion.
Alternatively or additionally, the line of detectors is parallel to the edge. Alternatively, the line of detectors is perpendicular to the edge. Alternatively, the line of detectors is neither parallel nor perpendicular to the edge.
In a preferred embodiment of the invention, each of the at least three detectors is situated at a different position with respect to the position of the edge such that each of the detectors detects a different amount of light depending on its relative position and wherein the computing circuitry detects the position based on the signals produced by the at least three detectors. Preferably, the magnitude of a change in position is determined based on movement of a characteristic derived from the signals produced by the at least three detectors.
Preferably, in any of the above described embodiments, the detectors are rectangular. Alternatively, the detectors are other than rectangular.
In a preferred embodiment of the invention, the detectors view a blurred representation of the edge. Alternatively, the detectors view a focused representation of the edge.
In a preferred embodiment of the invention, the apparatus includes at least one additional linear array of detectors, wherein said at least one additional array and said linear array form a two dimensional array of detectors. Alternatively or additionally, the at least one edge comprises a plurality of edges.
There is also provided in accordance with a preferred embodiment of the invention, apparatus for the measurement of position of an object in a given direction, comprising:
a plurality of light detectors each of which produces a signal at a given position in response to light reaching the detector;
a plurality of edges between dark area and light areas, at least one edge being viewed by at least one detector, which at least one edge moves, in relation to the detectors, as the object moves in the given direction; and
computing circuitry which receives signals from the plurality of detectors and determines the given position responsive to the signals produced by the detectors at the given position.
In a preferred embodiment of the invention, the computing circuitry computes a plurality of estimates of position based on different ones of the signals and which determines the position based on a plurality of said estimates.
There is also provided in accordance with a preferred embodiment of the invention, apparatus for the measurement of position of an object in a given direction, comprising:
a plurality of light detectors each of which produces a signal at a given position in response to light reaching the detector;
at least one edge between a dark area and a light area viewed by the plurality of detectors, which at least one edge moves, in relation to the detector, as the object moves in the given direction;
computing circuitry which computes a plurality of estimates of positions based on different ones of the signals produced by the detectors and which determines the given position based on a plurality of said estimates.
Preferably, the estimates are averaged to produce a determined position having an accuracy greater than the accuracy of the individual estimates.
In a preferred embodiment of the invention, the edges are edges of at least one strip. Preferably, the detectors have a first extent in the given direction and the strips have a second extent in the given direction and wherein the second extent is greater than the first extent.
There is also provided in accordance with a preferred embodiment of the invention, apparatus for the measurement of position of an object in a given direction, comprising:
a plurality of light detectors, having a first extent in the given direction, each of said detectors producing a signal at a given position in response to light reaching the detector;
at least one strip, having a second extent in the given direction, and having at least one edge between a dark area and a light area which is viewed by the plurality of detectors, which at least one edge moves, in relation to the detector, as the object moves in the given direction; and
computing circuitry which receives signals from the plurality of detectors and determines the given position responsive to the signals produced by the detectors at the given position,
the second extent being greater than the first extent.
Preferably, the at least one strip comprises a plurality of parallel strips. Alternatively or additionally, the plurality of strips have different widths or spacings and wherein the computing circuitry determines the strip with which the edge is associated from the width or spacing of the strip.
In a preferred embodiment of the invention, a light detector produces a first signal when it views a dark area and a second signal when it views a light area and the computing circuitry determines the position based on the strength of the signal generated by the detector at the given detector relative to the first and second signals.
Alternatively or additionally, the at least one edge is attached to the object and moves with it. Alternatively, the at least one edge is stationary and is viewed by a detector attached to the moving object.
Alternatively or additionally each of the plurality of detectors has a given extent in the given direction and wherein the computing circuitry determines the position to an accuracy greater than the extent. Preferably, the accuracy is at least 10 times greater than the extent of the detector. More preferably, the accuracy is at least 50 times greater than the extent of the detector. Most preferably, the accuracy is at least 100 times greater than the extent of the detector.
Alternatively or additionally, the computing circuitry determines the position of the object from signals produced only at that position.
In a preferred embodiment of the invention, the signals produced by the detector have a gray level scale of at least 10 gray levels and the gray level values are utilized in the determination of the position.
There is also provided in accordance with a preferred embodiment of the invention, apparatus for the measurement of position of an object in a given direction, comprising:
at least one light detector, said at least one detector producing a signal, having a gray level scale of at least 10 values, at a given position in response to light reaching the detector;
at least one edge, between a dark area and a light area which is viewed by the at least one detector, which at least one edge moves, in relation to the at least one detector, as the object moves in the given direction; and
computing circuitry which receives signals from the plurality of detectors and determines the given position responsive to the signal produced by the at least one detector at the given position, based on the gray level values and from signals produced only at that position.
In a preferred embodiment of the invention, the computing circuitry utilizes signals produced by only a portion of the detectors in the determination of the position.
Alternatively or additionally, the edges are formed in two groups of parallel edges wherein the first group of edges are regularly spaced from each other in the direction of motion and the second group of edges are regularly spaced from each other in the direction of motion and wherein the edges of the first and second groups are not regularly spaced from each other.
Alternatively or additionally, the light detector is sensitive to light of a given wavelength or wavelengths and wherein the position is determined to an greater accuracy than the diffraction limit of the light.
There is additionally provided for, in accordance with a preferred embodiment of the invention, apparatus for measurement of motion in two directions comprising:
apparatus according to any of the preceding claims for measurement of motion in one of the directions; and
apparatus according to any of the preceding claims for measurement of motion in the other of the two directions.
There is also provided for in accordance with a preferred embodiment of the invention, apparatus for measurement of position in two directions comprising:
an array of detectors having a plurality of detectors each of which produces a signal at a given position in response to light reaching the detector;
a first edge, between a dark area and a light area and having a first orientation, viewed by at least one of the plurality of detectors;
a second edge, between a dark area and a light area and having a second orientation, viewed by at least one of the plurality of detectors; and
computing circuitry which receives signals from the at least two detectors viewing the first and second edges and determines the position of the edge in the two directions, responsive to the received signals. Preferably, the array is a linear array. Alternatively, the array is a matrix array.
There is also provided in accordance with another preferred embodiment of the invention, apparatus for determination of position in two directions comprising:
a first edge between a bright area and a dark area, the edge being oriented in a first direction;
a second edge between a bright area and a dark area, the edge being oriented in a second direction and overlapping the first edge;
an array of detectors, each of which produces a signal in response to the light which it views, wherein the array views both the first and second edges; and
computing circuitry which computes the position in both directions responsive to the signals produced by the detectors.
Preferably, the apparatus includes means for selectively activating the edges such that when the first edge is activated the position in one direction is computed and when the other edge is activated the position in another direction is activated.
Alternatively, the dark areas associated with the first and second edges have a different color and the array of detectors includes at least one detector which responds selectively to one of the different colors.
There is therefore provided in accordance with another preferred embodiment of the invention, scanning apparatus comprising:
a scanning bed having:
at least two edges, each of said edges having a band of alternating dark and light strips running therealong; and
an area suitable for accepting a document to be scanned between said bands; and
a scanning unit having a linear array of optical detectors, each said detector producing a signal in response to light received by the detector, the unit being operative to be moved along the scanning bed, such that a first plurality of detectors view the document and a second and a third plurality of detectors view the two bands respectively.
Preferably, the apparatus includes computing circuitry which computes the position and orientation of the scanning unit relative to the scanning bed, responsive to the signals produced by the detectors. Alternatively or additionally, each of said bands and the plurality of detectors viewing said bands are comprised in apparatus according to any of the above described embodiments.
There is also provided in accordance with a preferred embodiment of the invention, a method of calibrating a position measurement which utilizes signals from a plurality of detectors which view at least one edge comprising:
measuring the signals of the plurality of detectors at a plurality of positions; and
calibrating the individual detectors based on averages of at least some of the measured signals.
The present invention will be more clearly understood from the following description of preferred embodiments of the invention in conjunction with the following drawings in which: